The prevalence of dementia among the elderly population is increasing, and Alzheimer's disease (AD) is a neurodegenerative disease that is the most common cause of dementia. Early detection of AD is critical for the effectiveness of a subsequent treatment of the same disease. Alzheimer's disease is usually diagnosed based on the person's medical history, family medical history, and behavioral observations. The presence of characteristic neurological and neuropsychological features and the absence of alternative conditions are supportive in diagnosis. Advanced medical imaging with computed tomography (CT) or magnetic resonance imaging (MRI), and with single-photon emission computed tomography (SPECT) or positron emission tomography (PET) can be used to help exclude other cerebral pathology or subtypes of dementia.
Medical organizations have created diagnostic criteria to ease and standardize the diagnostic process for practicing physicians. For example, the National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and the Alzheimer's Disease and Related Disorders Association (ADRDA, now known as the Alzheimer's Association) established the most commonly used NINCDS-ADRDA criteria for diagnosis of AD (McKhann G. et al. (1984), Neurology, 34:939-944).
However, the aforesaid diagnosis methods are time-consuming and rely on the physicians' experience. Therefore, it is necessary to search for biomarkers that can give rise to a quick, reliable and accurate diagnosis of AD.
Currently, most studies of AD biomarkers have mainly focused on known pathological substrates for the disease, such as amyloid plaques and neurofibrillary tangles, which are respectively composed of the abnormally aggregated amyloid-β peptide (Aβ) and hyperphosphorylated Tau (pTau). Several studies have shown that the major species of amyloid-β peptide, i.e. a 42-amino acid peptide (Aβ1-42), is significantly decreased in the cerebrospinal fluid (CSF) of patients with AD, and that pTau is elevated in the CSF of patients with AD. Although the studies exploring use of these two biomarkers in the diagnosis of disease have been carried out, the results have not led to a useful, definitive method.
The N-methyl-D-aspartate receptor (NMDAR) is a glutamate receptor and ion channel protein found in nerve cells. The NMDAR is activated when glutamate and glycine (or D-serine) are bound thereto. The activated NMDAR allows positively charged ions to flow through the cell membrane thereof. The NMDAR is critical for synaptic plasticity, memory and cognitive function. Attenuation of NMDAR-mediated neurotransmission results in loss of neuronal plasticity and cognitive deficits in the aging brain, which might account for clinical deterioration and brain atrophy.
There are several avenues to enhance NMDAR activation, and one of them is through inhibition of D-amino acid oxidase (DAAO), which is a flavoenzyme of peroxisomal enzyme responsible for degrading D-serine and D-alanine. The inhibition of DAAO thereby raises the level of D-amino acids which are neurotransmitters for the coagonist site of the NMDAR. The gene encoding D-amino acid oxidase activator (DAOA, also known as G72) is primate specific and is located on chromosome 13q32-q34. G72 protein may play an important role in the modulation of NMDA signaling. In a previous study, C. H. Lin et al. found that the peripheral G72 protein expression is distinctively higher in patients with schizophrenia than in healthy individuals, indicating that the peripheral expression of G72 protein may have the potential to be a diagnostic biomarker for schizophrenia (C. H. Lin et al. (2014), Molecular Psychiatry, 19:1-2).
The cystine/glutamate antiporter system xc− is a sodium-independent acidic amino acid transporter which mediates the uptake of cystine into cells in exchange for glutamate in a 1:1 ratio. System xc− is composed of a heavy chain subunit (4F2hc, SLC3A2) and a light chain subunit (xCT, SLC7A11). Cystine is reduced to cysteine intracellularly after being taken up by system xc−. Cysteine is the rate-limiting substrate for the synthesis of antioxidant glutathione (GSH) which is one of the most important antioxidants in the brain. System xc− also plays a critical role in the release of glutamate which is the most abundant amino acid neurotransmitter in the mammalian brain.
As far as the applicants are aware, the correlation of either G72 protein or System xc− with AD has yet to be understood. In order to explore a new biomarker for diagnosing AD, the applicants have conducted experiments and statistical analyses to determine the diagnostic accuracy of G72 protein and/or SLC7A11 mRNA in detection of AD. The applicants surprisingly found from experiments that even though G72 protein or SLC7A11 mRNA alone may be useful as a potential biomarker for the detection of AD, the combination of G72 protein and SLC7A11 mRNA can lead to an even more reliable diagnosis of AD.